In AC (alternating current) coupled systems, an AC coupling (i.e., capacitive coupling) or an inductive coupling (i.e., transformer coupling) behaves as a high pass filter, only allowing high frequency content to go through. The high pass filter behavior becomes a problem when the data traffic is not DC (direct current) balanced, meaning the traffic has a significant amount of low frequency content. For example, 16 G Fibre Channel (FC) uses 64b/66b coding, which is not DC balanced. The low frequency content is filtered out by the high pass filter behavior of the capacitive or inductive coupling. Baseline wander correction (BLWC) is used to address this problem.
Baseline wander also occurs in medical equipment where the low frequency loss can be due to poor contact. With respect to an electrocardiogram (ECG), the baseline wander is an extraneous, low-frequency activity, which may interfere with signal analysis, making the clinical interpretation inaccurate. When baseline wander takes place, ECG measurements related to the isoelectric line cannot be computed since the isoelectric line is not well-defined. Baseline wander in ECGs is often exercise-induced, and can come from a variety of sources, including perspiration, respiration, body movements and poor electrode contact. The spectral content of the baseline wander in an ECG is usually in the range between 0.05-1 Hz. However, during strenuous exercise, the baseline wander may contain higher frequencies.
A technique for correcting baseline wander is to low pass filter a received signal to restore the low frequency content and add the filtered signal back to the received signal. However, setting a gain for the baseline wander correction is not trivial. The optimal gain setting depends not only on the channel loss in the low frequency region, but also on the de-emphasis settings at the transmitter.
Conventional BLWC techniques involve manually setting the gain. Users either (i) set the gain based on a formula, (ii) sweep across a range of gain settings and pick one that works best, or (iii) find a starting point for the gain using an average of an eye envelope at the receiver input. Setting the gain manually is a significant problem for users, no matter what method is used. The optimal gain depends on the channel loss at the low frequency region, as well as the transmitter (TX) de-emphasis settings. Consequently, the gain needs to be different for different channels and needs to be updated when the TX de-emphasis changes.
It would be desirable to have an automatic adaptation of baseline wander correction loop gain settings.